Branch-Circuit, Feeder and Service Calculations, Part VI

Load calculation requirements are in Article 220 of the National Electrical Code (NEC). This article provides requirements for calculating branch-circuit, feeder and service loads. Article 220 is divided into five parts. Part I provides for general requirements for calculation methods. Part II provides calculation methods for branch-circuit loads. Parts III and IV provides calculation methods for feeder and services. Part V provides calculation methods for farms.

Understanding how to perform load calculations in accordance with the NEC is essential in an electrician’s professional career. Knowing what to do with the results of the load calculations is just as, if not more, important than the calculation. Other articles will provide additional specifications for finding overcurrent protection and conductor sizes for loads calculated in accordance with Article 220.

Last month’s column concluded by covering show windows in 220.14(G). This month, the discussion continues with more requirements for general-use receptacles and outlets not used for general illumination.

Fixed multioutlet assemblies are covered after show windows in the list of general-use receptacles and outlets not used for general illumination. Article 100 defines multioutlet assembly as a type of surface, flush or freestanding raceway designed to hold conductors and receptacles, assembled in the field or at the factory.

Most electricians are familiar with Plugmold, one type of multioutlet assembly. Plugmold is a registered trademark of Wiremold/Legrand. Multioutlet assemblies are manufactured in various lengths and receptacle outlet configurations (see Figure 1).

Fixed multioutlet assemblies used in other than dwelling units or the guest rooms or guest suites of hotels or motels must be calculated in accordance with either one of two methods in 220.14(H). For the purposes of this section, the calculation shall be permitted to be based on the portion that contains receptacle outlets.

Where appliances are unlikely to be used simultaneously, each 5 feet (1.5 m) or fraction thereof of each separate and continuous length shall be considered as one outlet of not less than 180 volt-amperes [220.14(H)(1)]. Providing the receptacle outlets in the multioutlet assembly will not be in use at the same time, the calculation can be based 180 volt-amperes for each 5 feet (1.5 m) or fraction thereof of each separate and continuous length.

For example, what is the calculated load for a 5-foot section of multioutlet assembly in a commercial occupancy? The receptacle outlets are unlikely to be used simultaneously. Because appliances are not likely to be used simultaneously, the 5-foot section is calculated at 180 volt-amperes (see Figure 2). The number of receptacle outlets in the multioutlet assembly does not factor into the calculation.

Where the length of a multioutlet assembly is 5 feet or less, it must be considered as one outlet of not less than 180 volt-amperes. Unlike other sections in Article 220 that use the phrase “major fraction,” this section specifies “fraction.” Therefore, after dividing the length by 5 feet, if the result contains a fraction, it must be rounded up to the next whole number.

For example, what is the calculated load for 21 feet of multioutlet assembly in a commercial occupancy? The receptacle outlets will not be in use at the same time. Since the outlets will not be in use at the same time, divide the length by 5 feet (21 ÷ 5 = 4.2). Because this number contains a fraction, increase 4.2 to 5. Finally, multiply 5 by 180 volt-amperes (5 x 180 = 900). The calculated load for 21 feet of multioutlet assembly in this example is 900 volt-amperes (see Figure 3).

Where appliances are likely to be used simultaneously, each foot (300 mm) or fraction thereof shall be considered as one outlet of not less than 180 volt-amperes [220.14(H)(2)]. Where receptacle outlets in the multioutlet assembly will likely be used at the same time, the calculation must be based on 180 volt-amperes for each foot or fraction thereof.

For example, a retail electronics store will have 58 feet of multioutlet assemblies installed. The receptacle outlets will be in use at the same time. What is the calculated load for 58 feet of multioutlet assemblies? Multiply the total length by 180 volt-amperes (58 x 180 = 10,440). The calculated load for 58 feet of multioutlet assembly in this example is 10,440 volt-amperes (see Figure 4).

Calculating loads for fixed multioutlet assemblies installed in dwelling units or guest rooms or guest suites of hotels or motels is not required. For example, 6 feet of multioutlet assembly will be installed in an attached garage of a single-family dwelling. The multioutlet assembly will be installed on the wall, above a work bench. What is the calculated load for 6 feet of multioutlet assembly in this garage? As specified in 220.14(H), no additional load calculation shall be required for multioutlet assemblies installed in dwelling units (see Figure 5). Note, because it will be installed in the garage of a dwelling, the multioutlet assembly must have ground-fault circuit interrupter (GFCI) protection for personnel [210.8(A)(2)].

Article 380 covers the use and installation requirements for multioutlet assemblies [380.1]. Article 380 is the econd shortest article in the NEC. Only three sections are covered in this article: scope, use and metal multioutlet assembly through dry partitions. The use of a multioutlet assembly is permitted in dry locations.

A multioutlet assembly shall not be installed as follows: 1. where concealed, except that it is permissible to surround the back and sides of a metal multioutlet assembly by the building finish or recess a nonmetallic multioutlet assembly in a baseboard, 2. where subject to severe physical damage, 3. where the voltage is 300 volts or more between conductors unless the assembly is of metal having a thickness of at least 0.040 in. (1.02 mm), 4. where subject to corrosive vapors, 5. in hoistways, and 6. in any hazardous (classified) locations except Class I, Division 2 locations as permitted in 501.10(B)(3) [380.2(B)] (see Figure 6).

Extending a metal multioutlet assembly through (not run within) dry partitions if arrangements are made for removing the cap or cover on all exposed portions and no outlet is located within the partitions is permissible [380.3] (see Figure 7).

Next month’s Code in Focus continues the discussion of load calculations. EC

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com, or www.charlesRmiller.com.